The present invention relates to a method of attaching an ultrasonic transducer to a surface for good acoustic transmission and to a novel ultrasonic coupling compound.
A coupling medium, such as a grease or the like, is frequently used to insure that ultrasonic energy can be transmitted between a transducer and the structure to which it is connected. It is necessary to use a coupling medium which will not degrade rapidly in its sonic conduction ability and which will remain in place in the space between the transducer and the surface to which it is connected. Ultrasonic transducers are frequently clamped to surfaces in an environment in which the coupling material will be subject to high temperature, vibration and other harsh environmental conditions. For example, "clamp-on" ultrasonic flowmeters which monitor fluid flow as disclosed in either of Baumoel patents 3,987,674 or 4,373,401 or fluid level as shown in Baumoel patent 4,144,517 may be subjected to temperatures of 500.degree. F. or higher due to extremely hot fluids within the conduit, e.g., liquid sodium. Conventional, commercially available ultrasonic couplants are unsuitable for such conditions. For example, the commonly available colloidal grease type couplants will exhibit an excessive degree of thermal outgassing with eventual loss of physical properties and loss of sonic coupling. Such changes would initially lead to constantly changing ultrasonic coupling which could generate erroneous ultrasonic data. Due to outgassing, chemical deterioration, or changed molecular cross-linking the in-service life of common high temperature couplants would be very unsatisfactory at such high temperatures. Because of this relatively short predicted service life, frequent changing of the couplant would be necessary. The cost of these changes would make use of such couplants prohibitive.
A typical prior art couplant material is DOW-CORNING 340 Heat Sink Compound. This material is believed to be described in U.S. Pat. No. 4,738,737 and is a grease-like silicone fluid heavily filled with zinc oxide, as an acoustic couplant material under high-temperature and high-radiation conditions. However, it has been found that such a material does not maintain its sonic properties at high temperatures (e.g., above 500.degree. F.) for a long enough time to avoid numerous changes of couplant.
Accordingly, there has been a long-standing need for a couplant which:
1. provides required acoustical properties in the form of a sound path with good acoustical impedance between a transducer and the pipe or other sonic medium to which it is attached;
2. withstands thermal cycling from room temperatures to temperatures over 500.degree. F., which are common as surfaces upon which sonic devices may be mounted;
3. maintains stable acoustical properties at temperatures over 500.degree. F., for periods of time which may be as great as several years between planned maintenance intervals;
4. emits a minimum of irritating fumes and does not outgas and disturb the sonic path;
5. has a viscosity which provides for ease of application and use; and
6. does not require expensive surface preparation of the conduit or pipe surfaces in the field, such as grinding or machining.
In search of a couplant which would answer all of these lone-standing needs, various materials which were available commercially for other uses were studied. The intent was to identify a material that did not have objectionably high contents of sulfur or lead or other toxic or noxious material; had the ability to perform the ultrasonic couplant function above 500.degree. F.; had a viscosity which was suitable for easy use as an ultrasonic couplant; and did not have hazardous or irritating fumes.
Fluoroether greases were identified as satisfactory couplant materials, particularly two specific fluoroether greases: Nye Fluoroether Grease 849 and Dupont Krytox.RTM. grease. These greases are known fluoroether lubricants; however, nothing suggests that fluorinated ethers, and particularly the Nye Fluoroether Grease 849 and Dupont Krytox.RTM. grease, might be an excellent ultrasonic acoustic couplant material in high temperature environments over 500.degree. F.